Any good store will have staff that should know their products.
when looking for a bed unless you really wish one a firm bed really isn't the way to go. A soft bed isn't necessarily bad it just may not be the right bed for you.
The support in any bed is mainly in the spring system. the pillow tops are just for comfort levels. Whether you decide on a spring system, an all latex mattress I would not recommend a water bed, it is a urban legend that it is good for a bad back as well as a firm being good for a bad back. Trust me I know!
These are questions you need to ask yourself and any salesperson worth their salt should bloody ask them too.
1. Do you have back problems, arthritis, had any surgery or going to have surgery in the near future etc.. Have you had any accidents, do you play a lot of sport if so what kind?
2. What are you use to sleeping on? How does it affect you? if you hurt where?
3. How old is your mattress?
4. How soon do you need your mattress and what is your budget?
5. Is that mattress just for you and your partner (if they have one, if the partner is not with them you ask all the questions in both cases ie: Do either of you have back problems etc) And if your partner is not with you it is advisable to bring your partner in to try with you if possible as the dynamics of a bed come into play with two or more people on a mattress.
6. Do you / or your partner toss and turn all night, are either of you a light sleeper. If one is a light sleeper and the other one tosses and turns they may keep their partner awake, in that case there are a few Mattress designs you could choose from that will help to minimise that so you both get a better sleep ie: individual pocket coils, an all latex mattress, a mattress with no coils, a tempur if you have them in your country etc.
These are all very important questions in finding the right Mattress for the individual/s.
I hope that helps you in some way. The right Mattress for you when you lay down should not cause you any pain. if it does get the hell off it and look for another one! I could go on and on but lets see what the sales person you find says.
Best of luck.. Finding the right mattress is very important for your body. Rest well.!
xx